A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage which may be time varying into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of the power switches of the power converter. The duty cycle is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50%). Additionally, as voltage or current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on a load microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to boost a time-varying input voltage, such as a rectified 120 V ac line voltage, to a higher, regulated output voltage, such as 400 V dc, to power a load. To provide the voltage conversion and regulation functions, the power converters include active power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the input voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of 100 kHz or higher.
Power converters coupled to an ac line are generally required to draw instantaneous line current waveform substantially proportional to the waveform of the ac line voltage. Such an arrangement substantially minimizes the root mean square (“RMS”) current drawn by the power converter for a given output power level. Control arrangements that draw a line current waveform proportional to the waveform of the ac line voltage are generally referred to as power factor controlled (“PFC”).
A controller for a power converter is generally formed as an integrated circuit with conductive pins that are soldered or otherwise electrically bonded to a printed wiring board in an end product. A design issue for power converters is the number of pins (physical circuit nodes) that is required for power converter control and for interactions with external system elements. For example, in a PFC control application a plurality of pins is generally required to sense an input line voltage, an input line current, and a power converter output voltage. The input line voltage is sensed for the PFC control function and to sense a brownout condition of the ac input line voltage so that the power converter can be protectively shut down at low input line voltages. The input line current is sensed for the PFC control function. The output voltage is also sensed for a protective and a regulation function. The utilization of a plurality of external pins to provide these functions incurs cost and physical space in a power converter design.
Several design approaches to providing power factor correction have been developed in recent years that have required progressively fewer pins. For example, some approaches avoid the need to accurately sense an input voltage or an input current. However, a practical implementation of a power converter employing power factor correction requires that the input line voltage be accurately sensed so that the power converter can be protected against an input voltage “brownout” condition wherein the RMS input line voltage is too low to support normal power converter operation. In addition, a practical implementation also requires that the power converter accurately sense output voltage to protect against an output overvoltage (or undervoltage) condition and for regulation of the output voltage. Accordingly, recent conventional design approaches have required the addition of physical pins to provide these protective and regulation features in a practical implementation.
Thus, there is a need for a process and related method to provide PFC control and protection functions for a switched-mode power converter with a minimal number of pins that avoids the disadvantages of conventional approaches.